N-acyl-hydrazine compounds as contrast enhancers for black-and-white photothermographic and thermographic elements

ABSTRACT

N-acyl-hydrazine compounds are useful as contrast enhancers when used in combination with (i) hindered phenol developers, and (ii) trityl hydrazide and/or formyl-phenyl hydrazine co-developers, to produce ultra-high contrast black-and-white photothermographic and thermographic elements. 
     The photothermographic and thermographic elements may be used as a photomask in a process where there is a subsequent exposure of an ultraviolet or short wavelength visible radiation-sensitive imageable medium.

BACKGROUND OF THE INVENTION

1. Field of Invention

N-Acyl-hydrazine compounds are useful as contrast enhancers when used in combination with (i) hindered phenol developers, and (ii) trityl hydrazide and/or formyl-phenyl hydrazinc co-developers, to produce ultra-high contrast black-and-white photothermographic and thermographic elements.

2. Background of the Art

Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) which are developed with heat, without liquid development have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a relatively non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example for the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.

The photosensitive compound is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those silver atoms are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag°) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. The silver halide may be made "in situ," for example by adding a halogen-containing source to the reducible silver source to achieve partial metathesis (see, for example, U.S. Pat. No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049). The silver halide may also be made "ex situ" and added to the organic silver salt. The addition of silver halide grains to photothermographic materials is described in Research Disclosure, June 1978, Item No. 17029. It is also reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely, so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.

The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.

In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag°). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further developed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms (i.e., the latent image). This produces a black-and-white image. In photographic elements, the silver halide is reduced to form the black-and-white image. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white image while much of the silver halide remains as silver halide and is not reduced.

In photothermographic elements the reducing agent for the organic silver salt, often referred to as a "developer," may be any material, preferably any organic material, that can reduce silver ion to metallic silver. At elevated temperatures, in the presence of the latent image, the silver ion of the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.

While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and cause fog during preparation and coating of photothermographic elements. As a result, hindered phenol reducing agents have traditionally been preferred.

As the visible image in black-and-white photothermographic and thermographic elements is usually produced entirely by elemental silver (Ag°), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic elements as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.

Another method of increasing the maximum image density in photothermographic and thermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating; dye-forming or dye-releasing materials in the emulsion. Upon imaging, the dye-forming or dye-releasing material is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced black,and-white silver image can be produced. Dye enhanced black-and-white silver image forming materials and processes are described in U.S. Pat. No. 5,185,231.

Thermographic imaging constructions (i.e., heat-developable materials) processed with heat, and without liquid development, are widely known in the imaging arts and rely on the use of heat to help produce an image. These elements generally comprise a support or substrate (such as paper, plastics, metals, glass, and the like) having coated thereon: (a) a thermally-sensitive, reducible silver source; (b) a reducing agent for the thermally-sensitive, reducible silver source (i.e., a developer); and (c) a binder.

In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing; agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, ascorbic acid derivatives, and the like, whereby an image of elemental silver is formed.

Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such instances, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of about 60°-225° C., resulting in the formation of an image.

The imaging arts have long recognized that the fields of photothermography and thermography are clearly distinct from that of photography. Photothermographic and thermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing.

In photothermographic imaging elements, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100° C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30°-50° C.).

In photothermographic elements only a small amount of silver halide is used to capture light and a different source of silver (e.g., silver behenate) is used to generate the image. Thus, in photothermography the silver halide serves as a silver catalyst generator for the development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed black-and-white photographic elements use only one form of silver (e.g., silver halide) which, upon development, is itself converted to the silver image. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.

Photothermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the reducing agent in developing the latent image. In contrast, photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the reduction of silver halide.

In photothermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions as compared to photographic emulsions. Even in so-called instant photography, developer chemistry is physically separated from the silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.

Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further exposure and spontaneous development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).

In photothermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.

Because photothermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending upon whether they have been incorporated in a photothermographic element or incorporated in a photographic element.

Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition); J. Sturge et al. Ed; Van Nostrand Reinhold: New York, 1989; Chapter 9 and in Unconventional Imaging Processes; E. Brinckman et al, Ed; The Focal Press: London and New York: 1978; pp. 74-75.

Hydrazides have been used in conventional wet processed black-and-white and color photographic systems. They have found use as nucleating agents, infectious developers, contrast and speed improving agents, and color developing agents.

Hydrazides have been studied as infectious developers for use in photographic graphic arts films. See U.S. Pat. Nos. 4,798,790 and 4,925,832 and Kitchin, J. P. et al. J. Photogr. Sci. 1987, 35, 162-164 and Kitchin, J. P. et al. J. Imag. Technol. 1989, 15(6), 282-284.

The use of hydrazides with hydroquinone-type (e.g., polyhydroxy benzene) developers is known to produce photographic elements with very high contrast in wet silver halide photographic materials. These materials have been found to be very advantageous for graphic arts laser imaging applications.

U.S. Pat. Nos. 4,891,295; 4,902,599; 4,952,474; 4,970,132; 5,037,719; 5,070,132; 5,196,288; and 5,246,811 all refer to the use of formyl-phenyl hydrazines and/or their derivatives in combination with traditional wet silver halide-type developers; silver halide; a polymerizable compound, and a dye or color-forming compound (sometimes contained in microcapsules). Upon imaging and heat development of the light-sensitive material, the hydrazide initiates polymerization within the area where the latent image has been formed. Pressure is then applied to transfer the dye image from the unpolymerized capsules to a receiving sheet. Silver soaps are optionally included and incorporation of the polymerization and dye-forming materials into microcapsules is preferred. In these systems, the hydrazide is an initiator for the polymerization while the developer develops the silver halide.

U.S. Pat. No. 4,735,884 uses trityl hydrazides in a gelatin-based construction as a reducing agent for silver halide as well as an initiator for the polymerization of acrylates within the area where the latent image has been formed. The hydrazides are encapsulated with other traditional silver halide developers, the photosensitive silver halide and leuco dyes or pre-formed dyes. After exposure, heat is applied to accelerate the crosslinking reactions, pressure is applied to break the non-exposed capsules, and the positive color image is transferred to a receiving layer.

U.S. Pat. No. 4,772,531 combines a sulfonamidophenol developing agent moiety and a trityl hydrazide group in one molecule to minimize the need for an additional developer within the polymerizaton system.

U.S. Pat. No. 3,667,958 uses a combination of a polyhydroxybenzene reducing agent (i.e., a hydroquinone-type developer) with a hydroxylamine, reductone and/or a hydrazinc reducing agent in photothermographic constructions to produce black-and-white images. A preferred hydrazine is 1-formyl-2-phenyl hydrazine.

U.S. Pat. application Ser. No. 08/369,730 (filed Jan. 6, 1995) discloses hydrazide compounds useful as co-developers for black-and-white photothermographic elements. These elements contain (i) a hindered phenol developer, and (ii) a trityl hydrazide or a fonnyl phenylhydrazine co-developer, and provide elements having high Dmax (>5.00), fast photospeeds, and high contrast (>20.0). It would be advantageous to improve the reactivity of these compounds by the addition of a contrast-promoting agent. This would allow the reduction in the amount of silver as well as reduction in the amount of hydrazine or hydrazide co-developer needed to achieve high contrast, lower costs, and improve the reactivity of less effective compounds. It would be especially desirable to be able to achieve in dry photothermographic or thermographic elements the high contrast that is currently available in wet silver halide materials. New developing agent systems for use in photothermographic and thermographic elements are therefore desired.

SUMMARY OF THE INVENTION

The present invention shows that a reducing agent system (i.e., a developer system) comprising: (i) at least one hindered phenol developer; (ii) at least one trityl hydrazide co-developer and/or at least one formyl-phenyl hydrazine co-developer; and (iii) a N-acyl-hydrazine compound, provides a black-and-white photothermographic and thermographic element having very high contrast and high image density (Dmax).

The present invention provides heat-developable, photothermographic and thermographic elements which are capable of providing high photospeeds, stable, high density images with high resolution, good sharpness, high contrast, and good shelf stability. The possibility of low absorbance at 350-450 nm facilitates the use of the elements of this invention in graphic arts applications such as contact printing, proofing, and duplicating ("duping").

The photothermographic elements of the present invention comprise a support bearing at least one photosensitive, image-forming, photothermographic emulsion layer comprising:

(a) a photosensitive silver halide;

(b) a non-photosensitive, reducible silver source;

(c) a reducing agent system for the non-photosensitive, reducible silver source; and

(d) a binder.

wherein the reducing agent system comprises:

(i) at least one hindered phenol developer;

(ii) at least one co-developer of the formula:

    R.sup.1 --(C═O)--NH--NH--R.sup.2

wherein:

R¹ represents hydrogen and R² represents an aryl group; or

R¹ represents hydrogen, alkyl or alkenyl groups of up to 20 carbon atoms, preferably alkyl or alkenyl groups of up to 10 carbon atoms, and more preferably of up to 5 carbon atoms; alkoxy, thioalkoxy, or amido groups of up to 20 carbon atoms, preferably of up to 10 carbon atoms, and more preferably of up to 5 carbon atoms; aryl, alkaryl, or aralkyl groups comprising up to 20 carbon atoms, preferably of up to 10 carbon atoms, and more preferably up to 6 carbon atoms; aryloxy, thioaryloxy, or anilino groups of up to 20 carbon atoms, preferably of up to 10 carbon atoms, and more preferably of up to 6 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups comprising up to 6 ring carbon atoms; or fused ring or bridging groups comprising up to 14 ring atoms; and R² represents a trityl group; and

(iii) at least one N-acyl-hydrazine compound of the formula ##STR1## wherein

R³ represents an alkyl or alkenyl group of up to 20 carbon atoms, preferably alkyl or alkenyl groups of up to 10 carbon atoms, and more preferably of up to 5 carbon atoms; alkoxy, thioalkoxy, or amido groups of up to 20 carbons, preferably up to 10 carbon atoms, and more preferably of up to 5 carbon atoms; aryl, alkaryl or aralkyl groups comprising up to 20 carbon atoms, preferably of up to 10 carbon atoms, and more preferably up to 6 carbon atoms; aryloxy, thioaryloxy, or anilino groups of up to 20 carbon atoms, preferably of up to 10 carbon atoms, and more preferably of up to 6 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups comprising up to 6 ring carbon atoms; or fused ring or bridging groups comprising up to 14 ring atoms.

When R² represents a trityl group, the compounds are referred to herein as trityl hydrazides. The phrase "trityl group" is often used to refer to the triphenylmethyl group (Ph₃ C--). As used herein, the phrase "trityl group" is broadened to include trihalomethyl (i.e., C--Ar₃), triphenylmethyl and substituted triphenylmethyl as well as triphenlylmethyl type groups wherein two of the phenyl groups are joined (for example, as in compound H-27 disclosed later herein).

When R¹ represents H and R² represents aryl or substituted aryl, the compounds are referred to herein as "formyl-phenyl hydrazincs" or "formyl-aryl hydrazines."

As used herein, "aryl" preferably represents phenyl or naphthyl.

When the photothermographic element used in this invention is heat developed, preferably at a temperature of from about 80° C. to about 250° C. (176° F. to 482° F.) for a duration of from about 1 second to about 2 minutes, in a substantially water-free condition after, or simultaneously with, imagewise exposure, a black-and-white silver image is obtained.

In photothermographic elements of the present invention, the layer(s) that contain the photographic silver salt are referred to herein as emulsion layer(s). According to the present invention, one or more components of the reducing agent system is added either to the emulsion layer(s) or to a layer or layers adjacent to the emulsion layer(s). Layers that are adjacent to the emulsion layer(s) may be, for example, protective topcoat layers, primer layers, interlayers, opacifying layers, antistatic layers, antihalation layers, barrier layers, auxiliary layers, etc. It is preferred that the reducing agent system be present in the photothermographic emulsion layer or topcoat layer.

The present invention also provides a process for the formation of a visible image by first exposing to electromagnetic radiation and thereafter heating the inventive photothermographic element.

The present invention also provides a process comprising the steps of:

(a) exposing the inventive photothermographic element on a support transparent to ultraviolet radiation or short wavelength visible radiation, to electromagnetic radiation to which the photosensitive silver halide of the: element is sensitive, to generate a latent image;

(b) heating the exposed element to develop the latent image into a visible image;

(c) positioning the element with a visible image thereon between a source of ultraviolet or short wavelength visible radiation energy and an ultraviolet or short wavelength radiation photosensitive imageable medium; and

(d) thereafter exposing; the imageable medium to ultraviolet or short wavelength visible radiation through the visible image on the element, thereby absorbing ultraviolet or short wavelength visible radiation in the areas of the element where there is a visible image and transmitting ultraviolet or short wavelength visible radiation through areas of the element where there is no visible image.

The photothermographic element may be exposed in step (a) with visible, infrared, or laser radiation.

The heat-developable, thermographic elements of the present invention comprise a support having coated thereon:

(a) a non-photosensitive, reducible silver source;

(b) a reducing agent system for the non-photosensitive, reducible silver source; and

(c) a binder;

wherein the reducing agent system comprises:

(i) at least one hindered phenol developer;

(ii) at least one co-developer of the formula:

    R.sup.1 --(C═O)--NH--NH--R.sup.2

wherein R¹ and R² are as defined above; and

(iii) at least one N-acyl-hydrazine compound of the formula: ##STR2## wherein R³ is as defined above.

In thermographic elements of the present invention, the layer(s) that contain the non-photosensitive reducible silver source are referred to herein as thermographic layer(s) or thermographic emulsion layer(s). When used in thermographic elements according to the present invention, one or more components of the reducing agent system is added either to the thermographic emulsion layer(s) or to a layer or layers adjacent to the emulsion layer(s). Layers that are adjacent to the emulsion layer(s) may be, for example, protective topcoat layers, primer layers, antistatic layers, interlayers, opacifying layers, barrier layers, auxiliary layers, etc. It is preferred that the reducing agent system be present in the thermographic layer or topcoat layer.

When the thermographic element used in this invention is heat developed, preferably at a temperature of from about 80° C. to about 250° C. (176° F. to 482° F.) for a duration of from about 1 second to about 2 minutes in a substantially water-free condition, a black-and-white silver image is obtained.

When used in a thermographic element, the image may be also be developed merely by heating at the above, noted temperatures using a thermal stylus or print head, or by heating while in contact with a heat absorbing material. Alternatively, the thermographic elements of the invention may also include a dye to facilitate direct development by exposure to laser radiation. Preferably the dye is an infrared absorbing dye and the laser is a diode laser emitting in the infrared. Upon exposure to radiation the radiation absorbed by the dye is converted to heat which develops the thermographic element.

The present invention also provides a process for the formation of a visible image by heating the inventive thermographic element described earlier herein.

The present invention further provides a process comprising the steps of:

(a) heating the inventive thermographic element on a support transparent to ultraviolet radiation or short wavelength visible radiation at a temperature sufficient to generate a visible image thereon;

(b) positioning the thermographic element with a visible image thereon between a source of ultraviolet or short wavelength visible radiation and an ultraviolet or short wavelength visible radiation photosensitive imageable medium; and

(c) thereafter exposing the imageable medium to ultraviolet or short wavelength visible radiation through the visible image on the element, thereby absorbing ultraviolet or short wavelength visible radiation in the areas of the element where there is a visible image and transmitting ultraviolet or short wavelength visible radiation through areas of the element where there is no visible image.

The reducing agent system (i.e., combination of developers and enhancers) used in this invention provide a significant improvement in image contrast when compared to photothermographic and thermographic elements incorporating known developers or developer combinations.

The photothermographic and thermographic elements of this invention may be used to prepare black-and-white images. The photothermographic material of this invention can be used, for example, in conventional black-and-white photothermography, in electronically generated black-and-white hardcopy recording, in the graphic arts area (e.g., phototypesetting), in digital proofing, and in digital radiographic imaging. The material of this invention provides high photospeeds, provides strongly absorbing black-and-white images, and provides a dry and rapid process.

Heating in a substantially water-free condition as used herein, means heating at a temperature of 80° to 250° C. The term "substantially water-free condition" means that the reaction system is approximately in equilibrium with water in the air, and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior to the element. Such a condition is described in T. H. James, The Theory of the Photographic Process, Fourth Edition, Macmillan 1977, page 374.

As used herein:

"photothermographic element" means a construction comprising at least one photothermographic emulsion layer and any supports, topcoat layers, image receiving layers, blocking layers;, antihalation layers, subbing or priming layers, etc;

"thermographic element" means a construction comprising at least one thermographic emulsion layer and any supports, topcoat layers, image receiving layers, blocking layers, antihalation layers, subbing or priming layers, etc;

"emulsion layer" means a layer of a photothermographic or thermographic element that contains the non-photosensitive silver source material and the photosensitive silver halide (when used);

"ultraviolet region of the spectrum" means that region of the spectrum less than or equal to about 400 nm, preferably from about 100 nm to about 400 nm. More preferably, the ultraviolet region of the spectrum is the region between about 190 nm and about 400 nm;

"short wavelength visible region of the spectrum" means that region of the spectrum from about 400 nm to about 450 nm; and

"infrared region of the spectrum" means from about 750 nm to about 1400 nm; "visible region of the spectrum" means from about 400 nm to about 750 nm; and "red region of the spectrum" means from about 640 nm to about 750 nm. Preferably the red region of the spectrum is from about 650 nm to about 700 nm.

R¹, R², and R³ in the foregoing-disclosed formulae may contain additional substituent groups. As is well understood in this area, substitution is not only tolerated, but is often advisable and substitution is anticipated on the compounds used in the present invention. As a means of simplifying the discussion and recitation of certain substituent groups, the terms "group" and "moiety" are used to differentiate between those chemical species that may be substituted and those which may not be so substituted. Thus, when the term "group," such as "aryl group," is used to describe a substituent, that substituent includes the use of additional substituents beyond the literal definition of the basic group. Where the term "moiety" is used to describe a substituent, only the unsubstituted group is intended to be included. For example, the phrase, "alkyl group" is intended to include not only pure hydrocarbon alkyl chains, such as methyl, ethyl, propyl, t-butyl, cyclohexyl, iso-octyl, octadecyl and the like, but also alkyl chains bearing substituents known in the art, such as hydroxyl, alkoxy, phenyl, halogen atoms (F, Cl, Br, and I), cyano, nitro, amino, carboxy, etc. For example, alkyl group includes ether groups (e.g., CH₃ --CH₂ --CH₂ --O--CH₂ --), haloalkyls, nitroalkyls, carboxyalkyls, hydroxyalkyls, sulfoalkyls, etc. On the other hand, the phrase "alkyl moiety" is limited to the inclusion of only pure hydrocarbon alkyl chains, such as methyl, ethyl, propyl, t-butyl, cyclohexyl, iso-octyl, octadecyl, and the like. Substituents that react with active ingredients, such as very strongly electrophilic or oxidizing substituents, would of course be excluded by the ordinarily skilled artisan as not being inert or harmless.

Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, examples, and claims.

DETAILED DESCRIPTION OF THE INVENTION

In photothermographic elements there exists the desire for products which exhibit increased contrast upon exposure to light and subsequent development. This desire is based upon the realization that contrast is directly related to the appearance of sharpness. Thus, products which exhibit increased contrast give the visual impression of enhanced sharpness.

Traditionally contrast has been defined by two methods, both of which are derived from the D-Log E curve. The first method is the determination of gamma, γ, which is defined as the slope of the straight-line section of the D-log E curve between two specified densities. The second is the determination of the overall sharpness of the toe section of the D-log E curve. By sharpness of the toe section, it is usually meant the relative change in density with exposure in the toe section of the traditional D-Log E curve. For instance, a sharp toe corresponds to a very rapid rise in density (at low levels of density) with exposure, whereas a soft toe corresponds to a very gradual rise in density (at low levels of density) with exposure. If either the value of γ is high or the toe is sharp, then the image has a relatively high contrast. If the value of γ is low, or the toe is soft, the image has a relatively low contrast. Contrast must be also be maintained throughout the exposure range. Thus, high γ at densities between about 2.0 and Dmax is also required to achieve sharp images.

The contrast must be optimized for each particular use. For some uses, certain parts of the sensitometric curve must be modified to increase or decrease the contrast of the product.

Photothermographic and thermographic systems have not found widespread use as replacement for wet silver halide in imaging systems because of slow speed, low Dmax, poor contrast, and insufficient sharpness at high Dmax. European Laid Open Patent Application No. 0 627 660 and U.S. Pat. No. 5,434,043 describe most of the characteristics and attributes of a photothermographic element having, for example, an antihalation system, silver halide grains having an average particle size of less than 0.10 μm, and infrared supersensitization leading to an infrared photothermographic article meeting the requirements for medical or graphic arts laser recording applications.

Conventional photothermographic elements comprising only bisphenol developers rarely exhibit a γ greater than about 3.0. These materials are well suited to medical imaging and similar uses where continuous tone reproduction is required but are not adequate for graphic arts uses where a much higher γ (e.g., >5.0) is necessary.

The shape of the sensitometric D-Log E curve for photothermographic elements of this invention is similar to that observed for infectious development curves in hard dot black-and-white conventionally processed wet silver halide image-setting films and allows the preparation of improved hard dot dry silver masks of high image quality useful for the production of plates in image-setting applications, contact proofs, and duplicating films also useful in the graphic arts. These masks are presently produced from conventional hybrid, hard dot, and rapid access wet silver halide materials.

The Reducing Agent System for the Non-Photosensitive Reducible Silver Source

In the black-and-white photothermographic and thermographic elements of the present invention, the reducing agent system (i.e., the developer system) for the organic silver salt comprises at least one hindered phenol compound; at least one co-developer of the formula:

    R.sup.1 --(C═O)--NH--NH--R.sup.2 ;

and at least one N-acyl-hydrazine compound of the formula: ##STR3## wherein R¹, R², and R³ are as defined earlier herein.

Hindered phenol developers are compounds that contain only one hydroxy group on a given phenyl ring and have at least one additional substituent located ortho to the hydroxy group. They differ from traditional photographic developers which contain two hydroxy groups on the same phenyl ring (such as is found in hydroquinones). Hindered phenol developers may contain more than one hydroxy group as long as each hydroxy group is located on different phenyl rings. Hindered phenol developers include, for example, binaphthols (i.e., dihydroxybinaphthyls), biphenols (i.e., dihydroxybiphenyls), bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)methanes, hindered phenols, and hindered naphthols.

Non-limiting representative binaphthols include 1,1'-bi-2-naphthol; 1,1'-bi-4-methyl-2-naphthol; and 6,6'-dibromo-bi-2-naphthol. For additional compounds see U.S. Pat. No. 5,262,295 at column 6, lines 12-13, incorporated herein by reference.

Non-limiting representative biphenols include 2,2'-dihydroxy-3,3'-di-t-butyl-5,5-dimethylbiphenyl; 2,2'-dihydroxy-3,3',5,5'-tetra-t-butylbiphenyl; 2,2'-dihydroxy-3,3'-di-t-butyl-5,5'-dichlorobiphenyl; 2-(2-hydroxy-3-t-butyl-5-methylphenyl)-4-methyl-6-n-hexylphenol; 4,4'-dihydroxy-3,3',5,5'-tetra-t-butylbiphenyl; and 4,4'-dihydroxy-3,3',5,5'-tetramethylbiphenyl. For additional compounds see U.S. Pat. No. 5,262,295 at column 4, lines 17-47, incorporated herein by reference.

Non-limiting representative bis(hydroxynaphthyl)methanes include 4,4'-methylenebis(2-methyl-1-naphthol). For additional compounds see U.S. Pat. No. 5,262,295 at column 6, lines 14-16, incorporated herein by reference.

Non-limiting representative bis(hydroxyphenyl)methanes include bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane (CAO-5); 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane (Permanax™); 1,1'-bis(3,5-tetra-t-butyl-4-hydroxy)methane; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 4,4-ethylidene-bis(2-t-butyl-6-methylphenol); and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane. For additional compounds see U.S. Pat. No. 5,262,295 at column 5, line 63, to column 6, line 8, incorporated herein by reference.

Non-limiting representative hindered phenols include 2,6-di-t-butylphenol; 2,6-di-t-butyl-4-methylphenol; 2,4-di-t-butylphenol; 2,6-dichlorophenol; 2,6-dimethylphenol; and 2-t-butyl-6-methylphenol.

Non-limiting representative hindered naphthols include 1-naphthol; 4-methyl-1-naphthol; 4-methoxy-1-naphthol; 4-chloro-1-naphthol; and 2-methyl-1-naphthol. For additional compounds see U.S. Pat. No. 5,262,295 at column 6, lines 17-20, incorporated herein by reference.

The co-developer may be a trityl hydrazide, a mixture of trityl hydrazides, a formyl-phenyl hydrazine, a mixture of formyl-phenyl hydrazines, or a mixture of at least one trityl hydrazide and at least one formyl-phenyl hydrazine.

Trityl hydrazides may be prepared by the reaction at room temperature of a stirred solution of a hydrazide and a slight molar excess of triethylamine dissolved in acetonitrile with an appropriately substituted trityl chloride (chlorotriphenylmethane). The trityl chloride is added gradually over a period of about 5 minutes. The mixture is warmed to 40° C. for approximately 1 hour. The mixture is poured into water and the resulting precipitate is filtered. In most cases, recrystallization of the solid from ethyl alcohol is appropriate.

Formyl-phenyl hydrazincs may be prepared by the addition of an appropriately substituted phenyl hydrazinc at 0° C. to a solution of formamide in acetic acid. After stirring for several hours at room temperature, the mixture is poured into water and the resulting precipitate is filtered. In most cases, recrystallization of the solid from ethyl alcohol is appropriate.

Representative trityl hydrazide and formyl-phenyl hydrazine co-developer compounds useful in the present invention are shown below. Preparation for these compounds are described later herein. These representations are exemplary and are not intended to be limiting. ##STR4##

The N-acyl-hydrazine contrast enhancer compounds may be prepared by the reaction at room temperature of a stirred solution of an ester with hydrazine. Many of these compounds are commercially available. Representative N-acyl-hydrazine contrast enhancer compounds useful in the present invention are shown below. These compounds are exemplary and are not intended to be limiting. ##STR5##

In the reducing agent system, the hindered phenol developer should be present at from 1 to 15% by weight of the imaging layer. When used, the trityl hydrazide co-developer should be present at from 0.1 to 15% by weight of the imaging layer. When used, the formyl-phenyl hydrazinc co-developer should be present at from 0.005 to 0.15% by weight of the imaging layer. The N-acyl-hydrazines should be present at from 0.1% to 10% by weight of the imaging layer.

The amounts of the above described reducing agents of the reducing agent system that are added to the photothermographic or thermographic element of the present invention may be varied depending upon the particular compound used, upon the type of emulsion layer, and whether components of the reducing agent system are located in the emulsion layer or a topcoat layer. However, when present in the emulsion layer, the hindered phenol should be present in an amount of from 0.01 to 50 mole, preferably from 0.05 to 25 mole; the trityl hydrazide should be present in an amount of from 0.0005 to 25 mole, preferably from 0.0025 to 10 mole; the formyl-phenyl hydrazide should be present in an amount of 0.0001 to 0.1 mole, preferably from 0.0005 to 0.05 mole; and the N-acyl-hydrazine should be present in an amount of from 0.0005 to 25 mole, preferably from 0.0025 to 10 mole --per mole of the silver halide.

In multilayer constructions, if one of the developers of the reducing agent system is added to a layer other than the emulsion layer, slightly higher proportions may be necessary and the hindered phenol should be present at from 2 to 20% by weight; the trityl hydrazide co.-developer when used should be present at from 0.2 to 20% by weight; the formyl-phenyl hydrazide when used should be present at from 0.01 to 0.20% by weight; and the N-acyl-hydrazine should be present in an amount of from 0.2% to 15% by weight of the layer in which it is present.

Photothermographic elements of the invention may also contain other additives such as shelf-life stabilizers, toners, development accelerators, acutance dyes, post-processing stabilizers or stabilizer precursors, and other image-modifying agents.

The Photosensitive Silver Halide

As noted above, when used in a photothermographic element, the present invention includes a photosensitive silver halide. The photosensitive silver halide can be any photosensitive silver halide, such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide, etc. The photosensitive silver halide can be added to the emulsion layer in any fashion so long as it is placed in catalytic proximity to the light-insensitive reducible silver compound which serves as a source of reducible silver.

The silver halide may be in any form which is photosensitive including, but not limited to cubic, octahedral, rhombic dodecahedral, orthorhombic, tetrahedral, other polyhedral habits, etc., and may have epitaxial growth of crystals thereon.

The silver halide grains may have a uniform ratio of halide throughout; they may have a graded halide content, with a continuously varying ratio of, for example, silver bromide and silver iodide; or they may be of the core-shell-type, having a discrete core of one halide ratio, and a discrete shell of another halide ratio. Core-shell silver halide grains useful in photothermographic elements and methods of preparing these materials are described in U.S. Pat. No. 5,382,504. A core-shell silver halide grain having an iridium doped core is particularly preferred. Iridium doped core-shell grains of this type are described in U.S. Pat. No.5,434,043.

The silver halide may be prepared ex situ, (i.e., be pre-formed) and mixed with the organic silver salt in a binder prior to use to prepare a coating solution. The silver halide may be pre-formed by any means, e.g., in accordance with U.S. Pat. No. 3,839,049. For example, it is effective to blend the silver halide and organic silver salt using a homogenizer for a long period of time. Materials of this type are often referred to as "pre-formed emulsions." Methods of preparing these silver halide and organic silver salts and manners of blending them are described in Research Disclosure, June 1978, item 17029; U.S. Pat. Nos. 3,700,458 and 4,076,539; and Japanese Pat. Application Nos. 13224/74, 42529/76, and 17216/75.

It is desirable in the practice of this invention to use pre-formed silver halide grains of less than 0.10 μm in an infrared sensitized, photothermographic material. It is also preferred to use iridium doped silver halide grains and iridium doped core-shell silver halide grains as disclosed in European Laid Open Pat. Application No 0 627 660 and U.S. Pat. No. 5,434,043 described above.

Pre-formed silver halide emulsions when used in the material of this invention can be unwashed or washed to remove soluble salts. In the latter case, the soluble salts can be removed by chill-setting and leaching or the emulsion can be coagulation washed, e.g., by the procedures described in U.S. Pat. Nos. 2,618,556; 2,614,928; 2,565,418; 3,241,969; and 2,489,341.

It is also effective to use an in situ process, i.e., a process in which a halogen-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide.

The light-sensitive silver halide used in the present invention can be employed in a range of about 0.005 mole to about 0.5 mole; preferably, from about 0.01 mole to about 0.15 mole per mole; and more preferably, from 0.03 mole to 0.12 mole of silver halide per mole of non-photosensitive reducible silver salt.

The silver halide used in the present invention may be chemically and spectrally sensitized in a manner similar to that used to sensitize conventional wet-processed silver halide or state-of-the-art heat-developable photographic materials.

For example, it may be chemically sensitized with a chemical sensitizing agent, such as a compound containing sulfur, selenium, tellurium, etc., or a compound containing gold, platinum, palladium, ruthenium, rhodium, iridium, or combinations thereof, etc., a reducing agent such as a tin halide, etc., or a combination thereof. The details of these procedures are described in T. H. James, The Theory of the Photographic Process, Fourth Edition, Chapter 5, pp. 149 to 169. Suitable chemical sensitization procedures are also disclosed in Shepard, U.S. Pat. No. 1,623,499; Waller, U.S. Pat. No. 2,399,083; McVeigh, U.S. Pat. No. 3,297,447; and Dunn, U.S. Pat. No. 3,297,446.

Addition of sensitizing dyes to the photosensitive silver halides serves to provide them with high sensitivity to visible and infrared light by spectral sensitization. Thus, the photosensitive silver halides may be spectrally sensitized with various known dyes that spectrally sensitize silver halide. Non-limiting examples of sensitizing dyes that can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanol dyes. Of these dyes, cyanine dyes, merocyanine dyes, and complex merocyanine dyes are particularly useful.

An appropriate amount of sensitizing dye added is generally about 10⁻¹⁰ to 10⁻¹⁰ mole; and preferably, about 10⁻⁸ to 10⁻³ moles of dye per mole of silver halide.

Supersensitizers

To get the speed of the photothermographic elements up to maximum levels and further enhance sensitivity, it is often desirable to use supersensitizers. Any supersensitizer can be used which increases the sensitivity. For example, preferred infrared supersensitizers are described in Eurpoean Laid Open Pat. Application No. 0 559 228 and include heteroaromatic mercapto compounds or heteroaromatic disulfide compounds of the formula:

    Ar--S--M

    Ar--S--S--Ar

wherein: M represents a hydrogen atom or an alkali metal atom.

In the above noted supersensitizers, Ar represents an aromatic ring or fused aromatic ring containing one or more of nitrogen, sulfur, oxygen, selenium or tellurium atoms. Preferably, the heteroaromatic ring is benzimidazole, naphthimidazole, benzothiazole, naphthothiazole, benzoxazole, naphthoxazole, benzoselenazole, benzotellurazole, imidazole, oxazole, pyrazole, triazole, thiadiazole, tetrazole, triazine, pyrimidine, pyridazine, pyrazine, pyridine, purine, quinoline or quinazolinone. However, other heteroaromatic rings are envisioned under the breadth of this invention.

The heteroaromatic ring may also carry substituents with examples of preferred substituents being selected from the group consisting of halogen (e.g., Br and Cl), hydroxy, amino, carboxy, alkyl (e.g., of 1 or more carbon atoms, preferably 1 to 4 carbon atoms) and alkoxy (e.g., of 1 or more carbon atoms, preferably of 1 to 4 carbon atoms.

Preferred supersensitizers are 2-mercaptobenzimidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzothiazole, and 2-mercaptobenzoxazole.

The supersensitizers are used in a general amount of at least 0.001 moles of sensitizer per mole of silver in the emulsion layer. Usually the range is between 0.001 and 1.0 moles of the compound per mole of silver and preferably between 0.01 and 0.3 moles of compound per mole of silver.

The Non-Photosensitive Reducible Silver Source Material

When used in photothermographic and thermographic elements, the present invention includes a non-photosensitive reducible silver source. The non-photosensitive reducible silver source that can be used in the present invention can be any material that contains a source of reducible silver ions. Preferably, it is a silver salt which is comparatively stable to light and forms a silver image when heated to 80° C. or higher in the presence of an exposed photocatalyst (such as silver halide) and a reducing agent.

Silver salts of organic acids, particularly silver salts of long chain fatty carboxylic acids, are preferred. The chains typically contain 10 to 30, preferably 15 to 28, carbon atoms. Suitable organic silver salts include silver salts of organic compounds having a carboxyl group. Examples thereof include a silver salt of an aliphatic carboxylic acid and a silver salt of an aromatic carboxylic acid. Preferred examples of the silver salts of aliphatic carboxylic acids include silver behenate, silver stearate, silver oleate, silver laurate, silver caprate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartarate, silver furoate, silver linoleate, silver butyrate, silver camphorate, and mixtures thereof, etc. Silver salts that can be substituted with a halogen atom or a hydroxyl group also can be effectively used. Preferred examples of the silver salts of aromatic carboxylic acid and other carboxyl group-containing compounds include: silver benzoate, a silver-substituted benzoate, such as silver 3,5-dihydroxybenzoate, silver o-methylbenzoate, silver m-methylbenzoate, silver p-methylbenzoate, silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silver p-phenylbenzoate, etc.; silver gallate; silver tannate; silver phthalate; silver terephthalate; silver salicylate; silver phenylacetate; silver pyromellilate; a silver salt of 3-carboxymethyl-4-methyl-4-thiazoline-2-thione or the like as described in U.S. Pat. No. 3,785,830; and a silver salt of an aliphatic carboxylic acid containing a thioether group as described in U.S. Pat. No. 3,330,663.

Silver salts of compounds containing mercapto or thione groups and derivatives thereof can also be used. Preferred examples of these compounds include: a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole; a silver salt of 2-mercaptobenzimidazole; a silver salt of 2-mercapto-5-aminothiadiazole; a silver salt of 2-(2-ethylglycolamido)benzothiazole; a silver salt of thioglycolic acid, such as a silver salt of a S-alkylthioglycolic acid (wherein the alkyl group has from 12 to 22 carbon atoms); a silver salt of a dithiocarboxylic acid such as a silver salt of dithioacetic acid; a silver salt of thioamide; a silver salt of 5-carboxylic-1-methyl-2-phenyl-4-thiopyridine; a silver salt of mercaptotriazine; a silver salt of 2-mercaptobenzoxazole; a silver salt as described in U.S. Pat. No. 4,123,274, for example, a silver salt of a 1,2,4-mercaptothiazole derivative, such as a silver salt of 3-amino-5-benzylthio-1,2,4-thiazole; and a silver salt of a thione compound, such as a silver salt of 3-(2-carboxyethyl)-4-methyl-4-thiazoline-2-thione as disclosed in U.S. Pat. No. 3,201,678.

Furthermore, a silver salt of a compound containing an imino group can be used. Preferred examples of these compounds include: silver salts of benzotriazole and substituted derivatives thereof, for example, silver methylbenzotriazole and silver 5-chlorobenzotriazole, etc.; silver salts of 1,2,4-triazoles or 1-H-tetrazoles as described in U.S. Pat. No. 4,220,709; and silver salts of imidazoles and imidazole derivatives.

Silver salts of acetylenes can also be used. Silver acetylides are described in U.S. Pat. Nos. 4,761,361 and 4,775,613.

It is also found convenient to use silver half soaps. A preferred example of a silver half soap is an equimolar blend of silver behenate and behenic acid, which analyzes for about 14.5% silver and which is prepared by precipitation from an aqueous solution of the sodium salt of commercial behenic acid.

Transparent sheet materials made on transparent film backing require a transparent coating. For this purpose a silver behenate full soap, containing not more than about 15% of free behenic acid and analyzing about 22% silver, can be used.

The method used for making silver soap emulsions is well known in the art and is disclosed in Research Disclosure, April 1983, item 22812, Research Disclosure, October 1983, item 23419, and U.S. Pat. No. 3,985,565.

The silver halide and the non-photosensitive reducible silver source material that form a starting point of development should be in catalytic proximity, i.e., reactive association. "Catalytic proximity" or "reactive association" means that they should be in the same layer, in adjacent layers, or in layers separated from each other by an intermediate layer having a thickness of less than 1 micrometer (1 μm). It is preferred that the silver halide and the non-photosensitive reducible silver source material be present in the same layer.

Photothermographic emulsions containing pre-formed silver halide in accordance with this invention can be sensitized with chemical sensitizers, or with spectral sensitizers as described above.

The source of reducible silver material generally constitutes about 5 to about 70% by weight of the emulsion layer. It is preferably present at a level of about 10 to about 50% by weight of the emulsion layer.

The Binder

The photosensitive silver halide, the non-photosensitive reducible source of silver, the reducing agent system, and any other addenda used in the present invention are generally added to at least one binder. The binder(s) that can be used in the present invention can be employed individually or in combination with one another. It is preferred that the binder be selected from polymeric materials, such as, for example, natural and synthetic resins that are sufficiently polar to hold the other ingredients in solution or suspension.

A typical hydrophilic binder is a transparent or translucent hydrophilic colloid. Examples of hydrophilic binders include: a natural substance, for example, a protein such as gelatin, a gelatin derivative, a cellulose derivative, etc.; a polysaccharide such as starch, gum arabic, pullulan, dextrin, etc.; and a synthetic polymer, for example, a water-soluble polyvinyl compound such as polyvinyl alcohol, polyvinyl pyrrolidone, acrylamide polymer, etc. Another example of a hydrophilic binder is a dispersed vinyl compound in latex form which is used for the purpose of increasing dimensional stability of a photographic element.

Examples of typical hydrophobic binders are polyvinyl acetals, polyvinyl chloride, polyvinyl acetate, cellulose acetate, polyolefins, polyesters, polystyrene, polyacrylonitrile, polycarbonates, methacrylate copolymers, maleic anhydride ester copolymers, butadiene-styrene copolymers, and the like. Copolymers, e.g., terpolymers, are also included in the definition of polymers. The polyvinyl acetals, such as polyvinyl butyral and polyvinyl formal, and vinyl copolymers such as polyvinyl acetate and polyvinyl chloride are particularly preferred.

Although the binder can be hydrophilic or hydrophobic, preferably it is hydrophobic in the silver containing layer(s). Optionally, these polymers may be used in combination of two or more thereof.

The binders are preferably used at a level of about 30-90% by weight of the emulsion layer, and more preferably at a level of about 45-85% by weight. Where the proportions and activities of the reducing agent system for the non-photosensitive reducible source of silver require a particular developing time and temperature, the binder should be able to withstand those conditions. Generally, it is preferred that the binder not decompose or lose its structural integrity at 250° F. (121° C.) for 60 seconds, and more preferred that it not decompose or lose its structural integrity at 350° F. (177° C.) for 60 seconds.

The polymer binder is used in an amount sufficient to carry the components dispersed therein, that is, within the effective range of the action as the binder. The effective range can be appropriately determined by one skilled in the art.

Photothermographic and Thermographic Formulations

The formulation for the photothermographic and thermographic emulsion layer can be prepared by dissolving and dispersing the binder, the photosensitive silver halide, (when used) the non-photosensitive reducible source of silver, the reducing agent system for the non-photosensitive reducible silver source, and optional additives, in an inert organic solvent, such as, for example, toluene, 2-butanone, or tetrahydrofuran.

The use of"toners" or derivatives thereof which improve the image, is highly desirable, but is not essential to the element. Toners can be present in an amount of about 0.01-10% by weight of the emulsion layer, preferably about 0.1-10% by weight. Toners are well known materials in the photothermographic and thermographic art, as shown in U.S. Pat. Nos. 3,080,254; 3,847,612; and 4,123,282.

Examples of toners include: phthalimide and N-hydroxyphthalimide; cyclic imides, such as succinimide, pyrazoline-5-ones, quinazolinone, 1-phenylurazole, 3-phenyl-2-pyrazoline-5-one, and 2,4-thiazolidinedione; naphthalimides, such as N-hydroxy-1,8-naphthalimide; cobalt complexes, such as cobaltic hexamine trifluoroacetate; mercaptans such as 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine, 3-mercapto-4,5-diphenyl-1,2,4-triazole and 2,5-dimercapto-1,3,4-thiadiazole; N-(aminomethyl)aryldicarboximides, such as (N,N-dimethylaminomethyl)phthalimide, and N-(dimethylaminomethyl)naphthalene-2,3-dicarboximide; a combination of blocked pyrazoles, isothiuronium derivatives, and certain photobleach agents, such as a combination of N,N'-hexamethylene-bis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-diazaoctane)bis(isothiuronium)trifluoroacetate, and 2-(tribromomethylsulfonyl benzothiazole); merocyanine dyes such as 3-ethyl-5-[(3-ethyl-2-benzothiazolinylidene)-1-methyl-ethylidene]-2-thio-2,4-o-azolidinedione; phthalazinone, phthalazinone derivatives, or metal salts or these derivatives, such as 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone, and 2,3-dihydro-1,4-phthalazinedione; a combination of phthalazine plus one or more phthalic acid derivatives, such as phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, and tetrachlorophthalic anhydride, quinazolinediones, benzoxazine or naphthoxazine derivatives; rhodium complexes functioning not only as tone modifiers but also as sources of halide ion for silver halide formation in situ, such as ammonium hexachlororhodate (III), rhodium bromide, rhodium nitrate, and potassium hexachlororhodate (III); inorganic peroxides and persulfates, such as ammonium peroxydisulfate and hydrogen peroxide; benzoxazine-2,4-diones, such as 1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione, and 6-nitro-1,3-benzoxazine-2,4-dione; pyrimidines and asym-triazines, such as 2,4-dihydroxypyrimidine, 2-hydroxy-4-aminopyrimidine, and azauracil; and tetrazapentalene derivatives, such as 3,6-dimercapto-1,4-diphenyl-1H, 4H-2,3a, 5,6a-tetraazapentalene and 1,4-di-(o-chlorophenyl)-3,6-dimercapto-1H, 4H-2,3a, 5,6a-tetraazapelntalene.

The photothermographic elements used in this invention can be further protected against the production of fog and can be stabilized against loss of sensitivity during storage. While not necessary for the practice of the invention, it may be advantageous to add mercury (II) salts to the emulsion layer(s) as an antifoggant. Preferred mercury (II) salts for this purpose are mercuric acetate and mercuric bromide.

Other suitable antifoggants and stabilizers, which can be used alone or in combination, include the thiazolium salts described in U.S. Pat. Nos. 2,131,038 and 2,694,716; the azaindenes described in U.S. Pat. No. 2,886,437; the triazaindolizines described in U.S. Pat. No. 2,444,605; the mercury salts described in U.S. Pat. No. 2,728,663; the urazoles described in U.S. Pat. No. 3,287,135; the sulfocatechols described in U.S. Pat. No. 3,235,652; the oximes described in British Patent No. 623,448; the polyvalent metal salts described in U.S. Pat. No. 2,839,405; the thiuronium salts described in U.S. Pat. No. 3,220,839; and palladium, platinum and gold salts described in U.S. Pat. Nos. 2,566,263 and 2,597,915. Stabilizer precursor compounds capable of releasing stabilizers upon application of heat during development can also be use in combination with the stabilizers of this invention. Such precursor compounds are described in, for example, U.S. Pat. Nos. 5,158,866, 5,175,081, 5,298,390, and 5,300,420.

Photothermographic and thermographic elements of the invention can contain plasticizers and lubricants such as polyalcohols and diols of the type described in U.S. Pat. No. 2,960,404; fatty acids or esters, such as those described in U.S. Pat. Nos. 2,588,765 and 3,121,060; and silicone resins, such as those described in British Patent No. 955,061.

Photothermographic and thermographic elements containing emulsion layers described herein may contain matting agents such as starch, titanium dioxide, zinc oxide, silica, and polymeric beads including beads of the type described in U.S. Patent Nos. 2,992,101 and 2,701,245.

Emulsions in accordance with this invention may be used in photothermographic and thermographic elements which contain antistatic or conducting layers, such as layers that comprise soluble salts, e.g., chlorides, nitrates, etc., evaporated metal layers, ionic polymers such as those described in U.S. Pat. Nos. 2,861,056, and 3,206,312 or insoluble inorganic salts such as those described in U.S. Pat. No. 3,428,451.

The photothermographic and thermographic elements of this invention may also contain electroconductive under-layers to reduce static electricity effects and improve transport through processing equipment. Such layers are described in U.S. Pat. No. 5,310,640.

Photothermographic Constructions

The photothermographic and thermographic elements of this invention may be constructed of one or more layers on a support. Single layer elements should contain the silver halide (when used), the non-photosensitive, reducible silver source material, the reducing agent system for the non-photosensitive reducible silver source, the binder as well as optional materials such as toners, acutance dyes, coating aids, and other adjuvants.

Two-layer constructions should contain silver halide (when used) and non-photosensitive, reducible silver source in one emulsion layer (usually the layer adjacent to the support) and the other ingredients in the second layer or distributed between both layers. Two layer constructions comprising a single emulsion layer coating containing all the ingredients and a protective topcoat are envisioned.

Photothermographic and thermographic emulsions used in this invention can be coated by various coating procedures including wire wound rod coating, dip coating, air knife coating, curtain coating, or extrusion coating using hoppers of the type described in U.S. Pat. No. 2,681,294. If desired, two or more layers can be coated simultaneously by the procedures described in U.S. Pat. Nos. 2,761,791; 5,340,613; and British Patent No. 837,095. Typical wet thickness of the emulsion layer can be about 10-150 micrometers (μm), and the layer can be dried in forced air at a temperature of about 20°-100° C. It is preferred that the thickness of the layer be selected to provide maximum image densities greater than 0.2, and, more preferably, in the range 0.5 to 4.5, as measured by a MacBeth Color Densitometer Model TD 504 using the color filter complementary to the dye color.

Photothermographic elements according to the present invention can contain acutance dyes and antihalation dyes. The dyes may be incorporated into the photothermographic emulsion layer as acutance dyes according to known techniques. The dyes may also be incorporated into antihalation layers according to known techniques as an antihalation backing layer, an antihalation underlayer or as an overcoat. It is preferred that the photothermographic elements of this invention contain an antihalation coating on the support opposite to the side on which the emulsion and topcoat layers are coated. Antihalation and acutance dyes useful in the present invention are described in U.S. Pat. Nos. 5,135,842; 5,226,452; 5,314,795.

Development conditions will vary, depending on the construction used, but will typically involve heating the imagewise exposed material at a suitably elevated temperature. When used in a photothermographic element, the latent image obtained after exposure can be developed by heating the material at a moderately elevated temperature of, for example, about 80°-250° C., preferably about 100°-200° C., for a sufficient period of time, generally about 1 second to about 2 minutes. Heating may be carried out by the typical heating means such as a hot plate, an iron, a hot roller, a heat generator using carbon or titanium white, or the like.

If desired, the imaged element may be subjected to a first heating step at a temperature and for a time sufficient to intensify and improve the stability of the latent image but insufficient to produce a visible image and later subjected to a second heating step at a temperature and for a time sufficient to produce the visible image. Such a method and its advantages are described in U.S. Pat. No. 5,279,928.

When used in a thermographic element, the image may be developed merely by heating at the above noted temperatures using a thermal stylus or print head, or by heating while in contact with a heat absorbing material.

Thermographic elements of the invention may also include a dye to facilitate direct development by exposure to laser radiation. Preferably the dye is an infrared absorbing dye and the laser is a diode laser emitting in the infrared. Upon exposure to radiation the radiation absorbed by the dye is converted to heat which develops the thermographic element.

The Support

Photothermographic and thermographic emulsions used in the invention can be coated on a wide variety of supports. The support, or substrate, can be selected from a wide range of materials depending on the imaging requirement. Supports may be transparent or at least translucent. Typical supports include polyester film, subbed polyester film (e.g.,polyethylene terephthalate or polyethylene naphthalate), cellulose acetate film, cellulose ester film, polyvinyl acetal film, polyolefinic film (e.g., polethylene or polypropylene or blends thereof), polycarbonate film and related or resinous materials, as well as glass, paper, and the like. Typically, a flexible support is employed, especially a polymeric film support, which can be partially acetylated or coated, particularly with a polymeric subbing or priming agent. Preferred polymeric materials for the support include polymers having good heat stability, such as polyesters. Particularly preferred polyesters are polyethylene terephthalate and polyethylene naphthalate.

Where the photothermographic or thermographic element is to be used as a photomask, the support should be transparent or highly transmissive of the radiation (i.e., ultraviolet or short wavelength visible radiation) which is used in the final imaging process.

A support with a backside resistive heating layer can also be used photothermographic imaging systems such as shown in U.S. Pat. No. 4,374,921.

Use as a Photomask

As noted above, the possibility of low absorbance of the photothermographic and thermographic element in the range of 350-450 nm in non-imaged areas facilitates the use of the photothermographic and thermographic elements of the present invention in a process where there is a subsequent exposure of an ultraviolet or short wavelength visible radiation sensitive imageable medium. For example, imaging the photothermographic or thermographic element and subsequent development affords a visible image. The developed photothermographic or thermographic element absorbs ultraviolet or short wavelength visible radiation in the areas where there is a visible image and transmits ultraviolet or short wavelength visible radiation where there is no visible image. The developed element may then be used as a mask and placed between an ultraviolet or short wavelength visible radiation energy source and an ultraviolet or short wavelength visible radiation photosensitive imageable medium such as, for example, a photopolymer, diazo material, or photoresist. This process is particularly useful where the imageable medium comprises a printing plate and the photothermographic or thermographic element serves as an imagesetting film.

Objects and advantages of this invention will now be illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.

EXAMPLES

All materials used in the following examples are readily available from standard commercial sources, such as Aldrich Chemical Co. Milwaukee, Wis., unless otherwise specified. All percentages are by weight unless otherwise indicated. The following additional terms and materials were used.

Acryloid™ A-21 is an acrylic copolymer available from Rohm and Haas, Philadelphia, Pa.

Butvar™ B-79 is a polyvinyl butyral resin available from Monsanto Company, St. Louis, Mo.

CAB 171-15S is a cellulose acetate butyrate resin available from Eastman Kodak Co.

CBBA is 2-(4-chlorobenzoyl)benzoic acid.

Desmodur™ N3300 is an aliphatic hexamethylene diisocyanate available from Bayer Chemicals, Pittsburgh, Pa.

2-MBI is 2-mercaptobenzimidazole.

2-MBO is 2-mercaptobenzoxazole.

2-MBT is 2-mercaptobenzthiazole.

MEK is methyl ethyl ketone (2-butanone).

MeOH is methanol.

MMBI is 2-mercapto-5-methylbenzimidazole.

4-MPA is 4-methylphthalic acid.

Permanax™ WSO is 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane [CAS RN=7292-14-0] and is available from St-Jean PhotoChemicals, Inc. Quebec. It is a reducing agent (i.e., a hindered phenol developer) for the non-photosensitive reducible source of silver. It is also known as Nonox™.

PET is polyethylene terephthalate.

PHP is pyridinium hydrobromide perbromide.

PHZ is phthalazine.

TCPA is tetrachlorophthalic acid.

Sensitizing Dye-1 is described in U.S. Pat. No. 5,441,866 and has the structure shown below ##STR6##

Antifoggant A is 2-(tribrornomethylsulfonyl)quinoline. Its preparation is disclosed in European Laid Open Pat. Application No 0 63 1176. It has the structure shown below. ##STR7##

Fluorinated Terpolymer A (FT-A) has the following random polymer structure, where m=7, n=2 and p=1. The preparation of fluorinated terpolymer A is described in U.S. Pat. No. 5,380,644. ##STR8##

Vinyl Sulfone-1 (VS-1) is described in European Laid Open Patent Application No. 0 600 589 A2 and has the following structure. ##STR9##

Antihalation Dye-1 (AH-1) has the following structure. The preparation of this compound is described in European Laid open Pat. Application No 0 342 810. ##STR10##

The samples were coated out under infrared safelights using a dual-knife coater. The photothermographic emulsion and and topcoat formulations were coated onto a 4 mil(101 μm) polyethylene terephthalate support provided with an antihalation back coating containing AH-1 in CAB 171-15S resin. After raising the hinged knives, the support was placed in position on the coater bed. The knives were then lowered and locked into place. The height of the knives was adjusted with wedges controlled by screw knobs and measured with electronic gauges. Knife #1 was raised to a clearance corresponding to the desired wet thickness of the support plus layer #1. Knife #2 was raised to a height equal to the desired wet thickness of the support plus layer #1 plus layer #2.

Aliquots of solutions #1 and #2 were simultaneously poured onto the support in front of the corresponding knives. The support was immediately drawn past the knives and into an oven to produce a double layered coating. The coated photothermographic or thermographic element was then dried by taping the support to a belt which was rotated inside a BlueM™ oven.

The following examples provide exemplary synthetic procedures for compounds of the invention. Photothermographic and thermographic imaging elements are shown.

Hydrazide Syntheses

General Procedure for the Synthesis of Trityl Hydrazides:

Procedure-A: A solution of 126 mmol of the appropriate hydrazide and 222 mmol of Et₃ N in 600 mL of acetonitrile was prepared at room temperature. To this was added 126 mmol of chlorolriphenylmethane. The mixture was then heated to 40° C. for approximately 2 hours. The mixture became very thick and stirring became somewhat difficult. The mixture was poured into water and filtered. The solid was then recrystallized from ethanol.

Procedure-B: A mixture of 27.6 g (99 mmol) of chlorotriphenylmethane in 130 mL of DMF was stirred vigorolasly. To this was added a solution of 13.1 g (99 mmol) of t-butyl carbazate and 15.8 g (200 mmol) of triethylamine in 20 mL of DMF dropwise. The mixture was stirred at room temperature for several hours. The mixture was filtered and while the filtrate was stirred vigorously, approximately 300 mL of ice water was added. The solid thus precipitated was filtered off and recrystallized from methanol.

Procedure-C: A solution of 30 mmol of the appropriate hydrazide and 30 mmol of chlorotriphenylmethane in 100 mL of pyridine was prepared and stirred. After stirring at room temperature for several hours, the mixture was poured into water. The solid was collected by filtration and washed thoroughly with water. The solid was then recrystallized from ethanol to provide the desired triphenylmethyl hydrazide.

Compound H-1 was prepared according to General Procedure-A described above, using 10.0 g of octanoic hydrazide, 8.8 g of triethylamine, 17.6 g of trityl chloride and 316 mL of acetonitrile. The normal workup procedure was followed and yielded 17.0 g (67%) of the desired product.

Compounds H-2, H-3, H-5, and H-25 were synthesized according to General Procedure-C.

Compound H-4 was prepared according to General Procedure-A using 10.0 g of benzoic hydrazide, 7.1 g of triethylamine, 20.5 g of trityl chloride and 150 mL of acetonitrile. The mixture was heated to 50° C. for 2 hours, then worked up in the normal fashion. The solid was then recrystallized to provide 11.5 g (41% yield) of the desired product.

Compound H-6 was prepared according to General Procedure-A, using 5.0 g of 3-hydroxybutyric acid hydrazinc, 4.35 g of triethylamine, 11.8 g of trityl chloride and 250 mL of acetonitrile. The mixture was heated for 2 hours then was cooled and poured into ice water. The solid then isolated was recrystallized from 1:1 hexane:ethyl acetate to provide the desired compound, m.p. 148°-150° C.

Compound H-7 was prepared according to General Procedure-A using 5.0 g of 4-nitrophenoxyacetic acid hydrazide, 4.0 g of triethylamine, 6.6 g of trityl chloride and 100 mL of acetonitrile. Normal workup and recrystallization provided 3.0 g (30% yield) of the desired product.

Compound H-8 was prepared according to General Procedure-A using 3.55 g of 2-thiophenecarboxylic acid, 3.16 g of triethylamine, 6.96 g of trityl chloride, and 150 mL of acetonitrile. The normal workup procedure was followed to provide 5.5 g (57% yield) of the desired compound, m.p. 185°-190° C.

Compound H-9 was prepared according to General Procedure-A using 10.0 g of isonicotinic acid hydrazide, 7.9 g of triethylamine, 20.3 g of trityl chloride and 400 mL of acetonitrile. The mixture was heated for 2 hours at 50° C. The normal workup procedure was followed to provide 10.7 g (37% yield) of the desired product as colorless, cubic crystals, m.p. 192°-194° C.

Compound H-10 was prepared according to General Procedure-B. t-Butyl carbazate (5.0 g) and of triethylamine (3.8 g) were dissolved in 45 mL of N,N-dimethylformamide (DMF). To this was added 10.5 g of trityl chloride in 4.5 mL of DMF. This was stirred overnight at room temperature then filtered. The remaining liquid was stirred vigorously while adding water slowly. A sticky yellowish mass resulted. The water was poured off and the solid was recrystallized from MeOH to provide the desired product as an off-white solid, 7.8 g (55% yield), m.p. 119°-122° C.

Compound H-11 was prepared according to General Procedure-A using 5.7 g of phenylacetylhydrazine, 3.6 g of triethylamine, 10.6 g of trityl chloride and 0 mL of acetonitrile. The mixture immediately became very thick with a white precipitate. The mixture was heated for 1 hour at 50° C., then the normal workup was followed. Recrystallization from 1:1 MeOH:EtOH provided 7.0 g of the desired product (47% yield).

Compound H-12 was prepared in an analogous manner to H-10.

Compounds H-13, H-20, H-21, H-22, H-23, H-24, H-26, H-27, and H-29 were also synthesized according to General Procedure-A from the appropriate hydrazine and appropriate trityl chloride.

Compound H-28 was prepared from methylthioacetyl hydrazine and triphenylmethyl chloride.

Preparation of Methylthioacetyl hydrazine: Ethyl methylthioacetate (40.0 g, 300 mmol) and hydrazine monohydrate (20.0 g, 400 mmol) were heated overnight at gentle reflux under nitrogen. Upon cooling, the solvent was removed at reduced pressure to provide methylthioacetyl hydrazine in nearly quantitative yield. ¹ H NMR and TLC indicated the material to be very pure. ¹ H NMR was in agreement with the desired structure.

Preparation of 1-(Methylthio)acetyl-2-triphenylmethylhydrazine: A solution of methylthioacetyl hydrazine prepared above (60.6 g, 500 mmol), Et₃ N and 1200 mL of acetonitrile was prepared and stirred at room temperature. To this solution was added triphenylmethyl chloride (trityl chloride; 139.4 g, 500 mmol). The reaction mixture became thick and difficult to stir. The magnetic stirrer was replaced with an overhead mechanical stirrer and stirring maintained for 0.5-1.0 hr. The reaction mixture was poured into water of equal volume to the solvent used. The solid was filtered off and the crude solid was recrystallized form approximately 2500 mL of hot ethanol to afford 1-(Methylthio)acetyl-2-triphenylmethylhydrazine (yield 45%; m.p. 149°-151° C.).

Synthesis of Formylphenylhydrazines:

Compound H-14 (1-Formyl-2-phenylhydrazine) was prepared by the method outlined in J. Chem. Soc. 1950, 3389. Phenylhydrazine 50.0 g (462 mmol) was added slowly to acetic acid 70 g (2.444 tool) maintained at 0° C. in an ice bath. Formamide (10.5 g, 466 retool) was added dropwise so as not to cause warming. The mixture gradually warmed to room temperature and was stirred for several hours. The mixture was poured into water and filtered. The solid was washed with very dilute acetic acid and then water. The crude material was dried and recrystallized from 200 mL of ethanol to provide 30 g (60% yield) of the desired compound as white, platelike crystals, m.p. 142°-44° C.

Compound H-15 was synthesized in a similar manner as described below for the preparation of H-17.

Compound H-16, is available from Aldrich Chemical Co.

Compound H-17 (1-Formyl-2-(4-methoxyphenyl)hydrazine) was prepared by addition of 5.0 g (28.8 retool) of 4-methoxyphenylhydrazine hydrochloride to a cooled solution of 18.0 g of acetic acid, followed by addition of 2.61 g (58.0 mmol) of formamide. The mixture was stirred for 3 hours and poured into water. The water was extracted with Et₂ O and the etheral layer separated and dried over Na₂ SO₄. Filtration and concentration of the solution afforded 1.65 g (38% yield) of the desired compound.

N-Acyl-hydrazine Syntheses

Compounds CA-1 to CA-5 are all commercially available.

Compound CA-6 was prepared by mixing 10.0 g (51.7 mmol) of ethyl bis(ethylthioacetate) with 3.0 g (60.0 retool) of hydrazine monohydrate. The solution was stirred at reflux overnight. The reaction was cooled and concentrated to provide a white solid.

The preparation of a pre-formed silver iodobromide emulsion, silver soap dispersion, homogenate, and halidized homogenate solutions used in the Examples is described below.

Formulation A- The following formulation was prepared to demonstrate the use of toners in the topcoat layer.

A pre-formed iridium-doped core-shell silver behenate full soap was prepared as described in U.S. Pat. No. 5,434,043 incorporated herein by reference.

The pre-formed soap contained 2.0% by weight of a 0.042 μm diameter iridium-doped core-shell silver iodobromide emulsion (25% core containing 8% iodide, 92% bromide; and 75% all-bromide shell containing 1×10⁻⁵ mole of iridium). A dispersion of this silver behenate full soap was homogenized to 23.0% solids in 2-butanone containing 1.1% Butvar™ B-79 polyvinyl butyral resin.

To 262.8 g of this silver behenate full soap dispersion was added 1.48 mL of a solution of 0.42 g of pyridinium hydrobromide perbromide in 2.35 g of methanol. After 1 hour of mixing 1.48 mL of a solution of 0.623 g of calcium bromide in 2.35 g of methanol was added. Mixing for 30 minutes was followed by addition of an infrared sensitizing dye solution prepared by mixing the following materials:

    ______________________________________                                         Material              Amount                                                   ______________________________________                                         CBBA                  3.588 g                                                  Sensitizing Dye-1     0.0735 g                                                 2-MBI                 0.0672 g                                                 2-MBO                 0.0252 g                                                 Methanol              12.088 g                                                 ______________________________________                                    

After 1 hour of mixing, the temperature was lowered from 70° F. to 55° F. Mixing for an additional 30 minutes at 55° F. was followed by addition of 47.7 g of Butvar™ B-79 polyvinyl butyral. With vigorous stirring for 60 minutes, the following components were added every 15 minutes.

    ______________________________________                                         Material            Amount                                                     ______________________________________                                         Antifoggant A       1.794 g                                                    Permanax ™       14.79 g                                                    Desmodur ™ N3300 1.215 g dissolved in                                       2-butanone          1.215 g                                                    PHZ                 1.479 g dissolved in                                       Methanol            1.75 g                                                     TCPA                0.666 g disolved in                                        2-butanone          2.625 g.                                                   ______________________________________                                    

A topcoat solution was prepared in the following manner; 1.29 g of Acryloid-21™ polymethyl methacrylate and 33.57 g of CAB 171-15S cellulose acetate butyrate were mixed in 404.7 g of 2-butanone and 53.4 g of methanol until dissolved. To 197.2 g of this prernix was then added 0.516 g of 4-methylphthalic acid, 0.928 g of a 15.0% solution of FT-A in ethyl acetate. The topcoat solution was diluted by the addition of 42.5 g of 2-butanone.

The photothermographic emulsion layer and topcoat layer were dual-knife coated onto a 4 mil (101 μm) polyester support containing an antihalation backcoat. The first knife gap for the photothermographic emulsion layer was set to 4.3 mil (109.2 μm) above the support and the second knife gap for the topcoat layer was set at 6.3 mil (160.02 μm) above the support. Samples were dried for 5 minutes at 185° F. (85.0° C.) in a BlueM™ oven as described above.

Formulation B: A pre-formed full soap was prepared, coated, and dried as described for Formulation A. The sensitizing dye solution was modified and was prepared by mixing the following materials:

    ______________________________________                                         Material              Amount                                                   ______________________________________                                         CBBA                  3.588 g                                                  Sensitizing Dye-1     0.0487 g                                                 MMBI                  0.134 g                                                  2-MBO                 0.0175 g                                                 Methanol              12.088 g                                                 ______________________________________                                    

The topcoat solution was also modified and was prepared as follows

    ______________________________________                                         Material              Amount                                                   ______________________________________                                         CAB solution          197.2 g                                                  4-MPA                 0.686 g                                                  FT-A                  1.856 g                                                  2-butanone            42.5 g                                                   VS-1                  0.196 g                                                  ______________________________________                                    

Sensitometry: The coated and dried photothermographic elements prepared from Formulations A, and B were cut into 1.5 inch×8 inch strips (3.8 cm×20.3 cm) and exposed with a laser sensitometer incorporating a 810 nm laser diode sensitometer for 6 seconds. The coatings were processed on a heated roll processor.

Densitometry measurements were made on a custom built computer scanned densitometer using a filter appropriate to the sensitivity of the photothermographic element and are believed to be comparable to measurements from commercially available densitometers.

Dmin is the density of the non-exposed areas after development. It is the average of eight lowest density values on the exposed side of the fiducial mark.

Dmax is the highest density value on the exposed side of the fiducial mark.

Speed-1 is Log1/E+4 corresponding to the density value of 1.00 above Dmin where E is the exposure in ergs/cm².

Speed-3 is Log1/E+4 corresponding to the density value of 3.00 above Dmin where E is the exposure in ergs/cm².

Contrast-A is the absolute value of the slope of the line joining the density points of 0.07 and 0.17 above Dmin.

Contrast-D is the absolute value of the slope of the line joining the density points of 1.00 and 3.00 above Dmin.

Contrast-E is the absolute value of the slope of the line joining the density points of 2.50 and 3.50 above Dmin.

Example 1

To 15 g aliquots of Formulation A was added 1.0 g of 2-butanone, 0.5 g of methanol, and one of the following:

4.3×10⁻⁵ moles CA-1;

1.71×10⁻⁴ moles CA-2;

1.60×10⁻⁴ moles H-8;

8.56×10⁻⁴ moles H-10;

a mixture of 4.3×10⁻⁵ moles of CA-1 and 1.60×10⁻⁴ moles of H-8;

a mixture of 4.3×10⁻⁵ moles of CA-1 and 8.56×10⁻⁴ moles of H-10;

a mixture of 1.71×10⁻⁴ moles of CA-2 and 1.60×10⁻⁴ moles of H-8;

a mixture of 1.71×10⁻⁴ moles of CA-2 and 8.56×10⁻⁴ moles of H-10;

A control sample containing only Permanax™, a hindered phenol developer was also evaluated.

The results, shown below, demonstrate that addition of N-acyl-hydrazine compounds increases the contrast and Dmax of the hydrazide/hindered phenol developer systems with only slight increase in Dmin.

    ______________________________________                                                                Processing                                              Ex.  Developer         Conditions                                                                               Dmin  Dmax                                    ______________________________________                                         1-1  Permanax ™     15 seconds/                                                                              0.20  4.43                                                           250° F.                                          1-2                    19 seconds/                                                                              0.22  4.02                                                           250° F.                                          1-3  Permanax ™ + CA-1                                                                             15 seconds/                                                                              0.19  4.19                                                           250° F.                                          1-4                    19 seconds/                                                                              0.20  4.00                                                           250° F.                                          1-5  Permanax ™ + H-8                                                                              15 seconds/                                                                              0.20  4.54                                                           250° F.                                          1-6                    19 seconds/                                                                              0.22  4.33                                                           250° F.                                          1-7  Permanax ™ + H-8 +                                                                            15 seconds/                                                                              0.21  5.00                                         CA-1              250° F.                                          1-8                    19 seconds/                                                                              0.24  5.03                                                           250° F.                                          1-9  Permanax ™ + CA-2                                                                             15 seconds/                                                                              0.19  4.31                                                           250° F.                                          1-10 Permanax ™ + H-8 +                                                                            15 seconds/                                                                              0.22  5.23                                         CA-2              250° F.                                          1-11 Permanax ™ + H-10                                                                             15 seconds/                                                                              0.20  4.28                                                           250° F.                                          1-12                   19 seconds/                                                                              0.23  4.22                                                           250° F.                                          1-13 Permanax ™ H-10 + CA-2                                                                        15 seconds/                                                                              0.21  4.89                                                           250° F.                                          1-14                   19 seconds/                                                                              0.23  4.76                                                           250° F.                                          ______________________________________                                         Ex.    Speed-1  Speed-3    Contrast-A                                                                             Contrast-D                                  ______________________________________                                         1-1    1.59     1.13       0.79    4.38                                        1-2    1.65     1.16       0.89    4.13                                        1-3    1.52     1.02       0.72    3.99                                        1-4    1.60     0.97       0.79    3.24                                        1-5    1.56     1.29       0.79    7.41                                        1-6    1.70     1.58       0.74    16.38                                       1-7    1.63     1.45       0.83    11.53                                       1-8    1.75     1.62       0.78    15.00                                       1-9    1.48     0.94       0.75    3.72                                        1-10   1.64     1.40       0.90    8.48                                        1-11   1.59     1.35       0.70    8.34                                        1-12   1.66     1.55       0.89    14.97                                       1-13   1.57     1.31       0.93    7.85                                        1-14   1.63     1.50       0.79    15.21                                       ______________________________________                                    

Example 2

To 15.0 aliquots of Formulation B silver premix was added 1.0 g 2-butanone and 0.5 g methanol, and one of the following:

8.52×10⁻⁵ moles CA-1;

8.53×10⁻⁵ moles CA-3;

8.53×10⁻⁵ moles CA-4;

1.70×10⁻⁴ moles CA-5;

4.28×10⁻⁴ moles H-10;

5.77×10⁻⁵ moles H-28;

a mixture of 5.77×10⁻⁵ moles H-28 and 8.52×10⁻⁵ moles CA-1;

a mixture of 5.77×10⁻⁵ moles H-28 and 8.53×10⁻⁵ moles CA-3;

a mixture of 5.77×10⁻⁵ moles H-28 and 1.70×10⁻⁴ moles CA-5;

a mixture of 4.28×10⁻⁴ moles H-10 and 8.52×10⁻⁵ moles CA-1;

a mixture of 4.28×10⁻⁴ moles H-10 and 8.53×10⁻⁵ moles CA-3;

a mixture of 4.28×10⁻⁴ moles H-10 and 8.53×10⁻⁵ moles CA-4;

a mixture of 4.28×10⁻⁴ moles H-10 and 1.70×10⁻⁴ moles CA-5.

A sample containing: only Permanax™ developer served as a control.

The sensitometric results, shown below, demonstrate that addition of N-acyl-hydrazine compounds to the hydrazide/hindered phenol developer system increases both contrast and Dmax. Contrast at densities greater than 2.0 was particularly increased (Contrast E).

    ______________________________________                                                                Processing                                              Ex.  Developer         Conditions                                                                               Dmin  Dmax                                    ______________________________________                                         2-1  Permanax ™     15 seconds/                                                                              0.25  4.46                                                           250° F.                                          2-2                    19 seconds/                                                                              0.29  4.37                                                           250° F.                                          2-3  Permanax ™ + CA-1                                                                             15 seconds/                                                                              0.26  4.60                                                           250° F.                                          2-4                    20 seconds/                                                                              0.30  4.44                                                           250° F.                                          2-5  Permanax ™ + CA-3                                                                             15 seconds/                                                                              0.25  4.29                                                           250° F.                                          2-6                    20 seconds/                                                                              0.29  4.10                                                           250° F.                                          2-7  Permanax ™ + CA-4                                                                             12 seconds/                                                                              0.26  4.46                                                           250° F.                                          2-8                    15 seconds/                                                                              0.28  4.55                                                           250° F.                                          2-9  Permanax ™ + CA-5                                                                             12 seconds/                                                                              0.24  4.42                                                           250° F.                                          2-10                   15 seconds/                                                                              0.26  4.34                                                           250° F.                                          2-11 Permanax ™ + H-28                                                                             12 seconds/                                                                              0.24  4.32                                                           250° F.                                          2-12                   15 seconds/                                                                              0.25  4.46                                                           250° F.                                          2-13 Permanax ™ + H-10                                                                             12 seconds/                                                                              0.24  4.49                                                           250° F.                                          2-14                   15 seconds/                                                                              0.26  4.41                                                           250° F.                                          2-15                   20 seconds/                                                                              0.30  4.42                                                           250° F.                                          2-16 Permanax ™ + H-28 +                                                                           15 seconds/                                                                              0.27  4.70                                         CA-1              250° F.                                          2-17 Permanax ™ + H-28 +                                                                           15 seconds/                                                                              0.27  4.44                                         CA-3              250° F.                                          2-18 Permanax ™ + H-28 +                                                                           12 seconds/                                                                              0.24  4.52                                         CA-5              250° F.                                          2-19                   15 seconds/                                                                              0.31  4.83                                                           250° F.                                          2-20 Permanax ™ + H-10 +                                                                           15 seconds/                                                                              0.26  4.58                                         CA-1              250° F.                                          2-21                   20 seconds/                                                                              0.32  4.40                                                           250° F.                                          2-22 Permanax ™ + H-10 +                                                                           15 seconds/                                                                              0.25  4.28                                         CA-3              250° F.                                          2-23                   20 seconds/                                                                              0.30  4.20                                                           250° F.                                          2-24 Permanax ™ + H-10 +                                                                           12 seconds/                                                                              0.25  4.50                                         CA-4              250° F.                                          2-25                   15 seconds/                                                                              0.29  4.63                                                           250° F.                                          2-26 Permanax ™ + H-10 +                                                                           12 seconds/                                                                              0.24  4.44                                         CA-5              250° F.                                          2-27                   15 seconds/                                                                              0.28  4.56                                                           250° F.                                          ______________________________________                                         Ex.  Speed-1  Speed-3  Contrast-A                                                                             Contrast-D                                                                             Contrast-E                              ______________________________________                                         2-1  1.58     1.19     0.88    5.18    2.57                                    2-2  1.65     1.16     0.68    4.06    2.49                                    2-3  1.60     1.19     0.76    4.88    2.61                                    2-4  1.66     1.19     0.68    4.24    2.25                                    2-5  1.58     1.10     0.77    4.15    2.23                                    2-6  1.65     1.10     0.79    3.66    1.68                                    2-7  1.46     1.06     0.86    5.02    3.24                                    2-8  1.59     1.19     0.87    5.08    2.86                                    2-9  1.48     1.06     0.86    4.80    3.32                                    2-10 1.57     1.15     0.83    4.75    2.48                                    2-11 1.49     1.11     0.85    5.23    3.15                                    2-12 1.59     1.42     0.76    11.64   5.55                                    2-13 1.50     1.16     0.81    5.85    3.84                                    2-14 1.60     1.41     0.80    10.47   2.12                                    2-15 1.72     1.64     0.85    24.70   1.36                                    2-16 1.62     1.49     0.85    14.95   9.89                                    2-17 1.61     1.46     0.91    14.28   11.94                                   2-18 1.50     1.25     0.74    7.86    5.69                                    2-19 1.68     1.56     0.91    16.72   21.18                                   2-20 1.63     1.48     0.76    13.98   6.16                                    2-21 1.76     1.69     0.80    29.04   38.78                                   2-22 1.60     1.44     0.78    12.89   1.57                                    2-23 1.72     1.65     0.81    27.25   19.52                                   2-24 1.49     1.17     0.85    6.11    4.79                                    2-25 1.60     1.47     0.90    15.43   16.60                                   2-26 1.50     1.18     0.90    6.31    4.23                                    2-27 1.65     1.53     0.82    17.96   24.43                                   ______________________________________                                    

Example 3

To 15.0 g aliquots of Formulation B silver premix was added 1.0 g 2-butanone and 0.5 g methanol, and one of the following:

4.26×10⁻⁵ moles CA-4;

8.53×10⁻⁵ moles CA-5;

5.77×10⁻⁵ moles H-28;

8.50×10⁻⁶ moles H-14;

4.28×10⁻⁴ moles H-10;

a mixture of 8.50×10⁻⁶ moles H-14 and 8.53×10⁻⁵ moles CA-5;

a mixture of 4.28×10⁻⁴ moles H-10 and 4.26×10⁻⁵ moles CA-4

a mixture of 4.28×10⁻⁴ moles H-10 and 8.53×10⁻⁵ moles CA-5;

a mixture of 5.77×10⁻⁵ moles H-28 and 8.53×10⁻⁵ moles CA-5.

The sensitometric results, shown below, demonstrate that addition of N-acyl-hydrazine compounds to the hydrazide/hindered phenol developer system increases both contrast and Dmax. Contrast at densities greater than 2.0 was particularly increased (Contrast E).

    ______________________________________                                                                Processing                                              Ex.  Developer         Conditions                                                                               Dmin  Dmax                                    ______________________________________                                         3-1  Permanax ™ + CA-4                                                                             20 seconds/                                                                              0.29  4.26                                                           245° F.                                          3-2                    13 seconds/                                                                              0.25  4.33                                                           250° F.                                          3-3  Permanax ™ + CA-5                                                                             20 seconds/                                                                              0.29  4.43                                                           245° F.                                          3-4                    13 seconds/                                                                              0.26  4.64                                                           250° F.                                          3-5                    15 seconds/                                                                              0.29  4.58                                                           250° F.                                          3-6  Permanax ™ + H-14                                                                             20 seconds/                                                                              0.28  4.97                                                           245° F.                                          3-7                    13 seconds/                                                                              0.25  4.54                                                           250° F.                                          3-8  Permanax ™ + H-14 +                                                                           20 seconds/                                                                              0.28  5.13                                         CA-5              245° F.                                          3-9                    13 seconds/                                                                              0.26  4.99                                                           250° F.                                          3-10 Permanax ™ + H-10                                                                             20 seconds/                                                                              0.27  4.31                                                           245° F.                                          3-11                   13 seconds/                                                                              0.25  4.22                                                           250° F.                                          3-12                   15 seconds/                                                                              0.27  4.36                                                           250° F.                                          3-13 Permanax ™ + H-10 +                                                                           20 seconds/                                                                              0.30  4.18                                         CA-4              245° F.                                          3-14                   13 seconds/                                                                              0.26  4.19                                                           250° F.                                          3-15 Permanax ™ + H-10 +                                                                           20 seconds/                                                                              0.27  4.25                                         CA-5              245° F.                                          3-16                   15 seconds/                                                                              0.28  4.44                                                           250° F.                                          3-17 Permanax ™ + H-28                                                                             20 seconds/                                                                              0.29  4.53                                                           245° F.                                          3-18                   15 seconds/                                                                              0.28  4.62                                                           250° F.                                          3-19 Permanax ™ + H-28 +                                                                           20 seconds/                                                                              0.29  4.77                                         CA-5              245° F.                                          3-20                   15 seconds/                                                                              0.30  5.15                                                           250° F.                                          ______________________________________                                         Ex.  Speed-1  Speed-3  Contrast-A                                                                             Contrast-D                                                                             Contrast-E                              ______________________________________                                         3-1  1.53     1.01     0.68    3.87    2.35                                    3-2  1.42     0.93     0.73    4.12    2.67                                    3-3  1.51     1.09     0.77    5.03    2.63                                    3-4  1.43     1.07     0.76    4.93    2.99                                    3-5  1.57     1.12     0.77    4.51    2.61                                    3-6  1.65     1.51     0.82    14.30   15.77                                   3-7  1.46     1.16     0.75    6.67    5.88                                    3-8  1.72     1.60     0.88    16.41   24.02                                   3-9  1.58     1.34     0.87    6.53    5.54                                    3-10 1.55     1.38     0.80    12.22   1.32                                    3-11 1.42     1.10     0.68    6.19    1.82                                    3-12 1.59     1.43     0.72    13.30   1.65                                    3-13 1.55     1.43     0.76    16.78   1.28                                    3-14 1.46     1.22     0.78    8.29    5.38                                    3-15 1.60     1.48     0.68    16.95   7.79                                    3-16 1.60     1.48     0.72    17.68   13.80                                   3-17 1.60     1.47     0.75    15.90   13.77                                   3-18 1.53     1.42     0.85    18.02   25.28                                   3-19 1.62     1.52     0.67    21.25   22.47                                   3-20 1.68     1.57     0.90    18.90   25.74                                   ______________________________________                                    

Example 4

To 15.0 g aliquots of Formulation B silver premix was added 1.0 g of 2-butanone, 0.5 g of methanol, and one of the following:

2.14×10⁻⁴ moles CA-6;

4.28×10⁻⁴ moles H-10;

8.5×10⁻⁶ moles H-14;

5.77×10⁻⁵ moles H-28;

a mixture of 4.28×10⁻⁴ moles H-10 and 2.14×10⁻⁴ moles CA-6;

a mixture of 8.50×10⁻⁶ moles H-14 and 2.14×10⁻⁴ moles CA-6;

a mixture of 5.77×10⁻⁵² moles H-28 and 2.14×10⁻⁴ moles CA-6.

A sample containing only Permanax™ developer served as a control.

The results, shown below, demonstrate that addition of N-acyl-hydrazine compounds increases the contrast and Dmax of the hydrazide/hindered phenol developer systems with only slight increase in Dmin.

    ______________________________________                                                                Processing                                              Ex.  Developer         Conditions                                                                               Dmin  Dmax                                    ______________________________________                                         4-1  Permanax ™     11 seconds/                                                                              0.23  3.56                                                           245° F.                                          4-2                    11 seconds/                                                                              0.24  4.39                                                           250° F.                                          4-3                    15 seconds/                                                                              0.27  4.36                                                           250° F.                                          4-4  Permanax ™ + CA-6                                                                             11 seconds/                                                                              0.24  3.76                                                           245° F.                                          4-5                    15 seconds/                                                                              0.24  3.93                                                           245° F.                                          4-6                    9 seconds/                                                                               0.23  3.55                                                           250° F.                                          4-7                    11 seconds/                                                                              0.24  4.05                                                           250° F.                                          4-8                    15 seconds/                                                                              0.26  4.04                                                           250° F.                                          4-9  Permanax ™ + H-10                                                                             11 seconds/                                                                              0.24  4.22                                                           250° F.                                          4-10                   15 seconds/                                                                              0.25  4.11                                                           250° F.                                          4-11 Permanax ™ + H-10 +                                                                           11 seconds/                                                                              0.24  4.34                                         CA-6              250° F.                                          4-12                   15 seconds/                                                                              0.27  4.30                                                           250° F.                                          4-13 Permanax ™ + H-14                                                                             11 seconds/                                                                              0.24  4.82                                                           245° F.                                          4-14                   9 seconds/                                                                               0.24  5.00                                                           250° F.                                          4-15 Permanax ™ + H-14 +                                                                           11 seconds/                                                                              0.25  5.20                                         CA-6              245° F.                                          4-16                   9 seconds/                                                                               0.24  5.27                                                           250° F.                                          4-17 Permanax ™ + H-28                                                                             15 seconds/                                                                              0.25  4.33                                                           245° F.                                          4-18                   11 seconds/                                                                              0.24  4.69                                                           250° F.                                          4-19 Permanax ™ + H-28 +                                                                           15 seconds/                                                                              0.25  4.42                                         CA-6              245° F.                                          4-20                   11 seconds/                                                                              0.25  4.78                                                           250° F.                                          ______________________________________                                         Ex.  Speed-1  Speed-3  Contrast-A                                                                             Contrast-D                                                                             Contrast-E                              ______________________________________                                         4-1  1.20     0.42     0.54    2.58                                            4-2  1.41     1.00     0.77    4.81    2.97                                    4-3  1.50     0.98     0.76    3.87    2.16                                    4-4  1.24     0.51     0.70    2.75    1.71                                    4-5  1.42     0.84     0.82    3.41    1.61                                    4-6  1.19     0.29     0.67    2.22                                            4-7  1.38     0.75     0.73    3.20    2.02                                    4-8  1.49     0.88     0.70    3.26    1.40                                    4-9  1.42     1.01     0.79    4.87    2.81                                    4-10 1.52     1.31     0.67    9.57    1.11                                    4-11 1.43     1.17     0.75    7.62    5.58                                    4-12 1.59     1.48     0.80    17.74   11.81                                   4-13 1.28     0.95     0.67    6.15    6.26                                    4-14 1.33     1.01     0.70    6.30    6.37                                    4-15 1.45     1.29     0.78    13.13   12.30                                   4-16 1.52     1.36     0.73    12.75   16.44                                   4-17 1.44     1.07     0.77    5.57    3.02                                    4-18 1.42     1.14     0.74    7.21    5.44                                    4-19 1.45     1.21     0.73    8.42    6.72                                    4-20 1.48     1.30     0.76    10.70   8.54                                    ______________________________________                                    

Reasonable modifications and variations are possible from the foregoing disclosure without departing from either the spirit or scope of the present invention as defined by the claims. 

What is claimed is:
 1. A photothermographic element comprising a support bearing at least one photosensitive, image-forming, photothermographic emulsion layer comprising:(a) a photosensitive silver halide; (b) a non-photosensitive, reducible silver source; (c) a reducing agent system for silver ion; and (d) a binder;wherein said reducing agent system comprises: (i) at least one hindered phenol; (ii) at least one co-developer of the formula

    R.sup.1 --(C═O)--NHNH--R.sup.2

wherein:R¹ represents hydrogen and R² represents an aryl or substituted aryl group; or, R¹ represents hydrogen, alkyl, or alkenyl groups of 1 to 20 carbon atoms; alkoxy, thioalkoxy, or amido groups of 1 to 20 carbon atoms; aryl, alkaryl, or aralkyl groups of up to 20 carbon atoms; aryloxy, thioaryloxy, or anilino groups of up to 20 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups comprising up to 6 ring carbon atoms; or fused ring or bridging groups comprising up to 14 ring atoms; and R² represents a trityl group; and (iii) at least one N-acyl-hydrazine compound of the formula ##STR11## wherein: R³ represents an alkyl or alkenyl groups of 1 to 20 carbon atoms; alkoxy, thioalkoxy, or amido groups of up to 20 carbons; aryl, alkaryl or aralkyl groups comprising up to 20 carbon atoms; aryloxy, thioaryloxy, or anilino groups of up to 20 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups comprising up to 6 ring carbon atoms; or fused ring or bridging groups comprising up to 14 ring atoms.
 2. The photothermographic element according to claim 1 wherein said non-photosensitive, reducible source of silver is a silver salt of a carboxylic acid having from 10 to 30 carbon atoms.
 3. The photothermographic element according to claim 1 wherein R¹ represents an alkyl or alkenyl group of 1 to 10 carbon atoms; an alkoxy, thioalkoxy, or amido group of 1 to 10 carbon atoms; an aryl, alkaryl, or aralkyl group of up to 10 carbon atoms; or an aryloxy, thioaryloxy, and amido group of up to 10 carbon atoms; and R² represents a trityl group.
 4. The photothermographic element according to claim 1 wherein R¹ represents an alkyl or alkenyl group of 1 to 5 carbon atoms; an alkoxy, thioalkoxy, or amido group of 1 to 5 carbon atoms; an aryl, alkaryl, or aralkyl group of up to 6 carbon atoms; or an aryloxy, thioaryloxy, or amido group of up to 6 carbon atoms; and R² represents a trityl group.
 5. The photothermographic element according to claim 1 wherein R¹ represents hydrogen and R² represents a phenyl group.
 6. The photothermographic element according to claim 1 wherein R¹ represents hydrogen or an α-thioether group and R² represents C-Ph₃.
 7. The photothermographic element according to claim 1 wherein said co-developer is a mixture of at least one trityl hydrazide and at least one formylphenyl hydrazine.
 8. The photothermographic element according to claim 1 wherein said binder is a hydrophobic synthetic polymer.
 9. The photothermographic element according to claim 1 wherein said non-photosensitive silver source is silver behenate.
 10. The photothermographic element of claim 1 wherein said hindered phenol is selected from the group consisting of binaphthols, biphenols, bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)methanes, and hindered naphthols.
 11. The photothermographic element of claim 10 wherein said hindered phenol is a bis(hydroxyphenyl)methane.
 12. The photothermographic element according to claim 1 wherein:R³ represents alkyl or alkenyl groups of 1 to 10 carbon atoms; alkoxy, alkylthio, or amido groups of 1 to 10 carbon atoms; aryl, alkaryl, or aralkyl groups of up to 10 carbon atoms; aryloxy, arylthio, or anilino groups of up to 10 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups of up to 6 ring carbon atoms; or fused ring or bridging groups of up to 14 ring atoms.
 13. The photothermographic element according to claim 1 wherein:R³ represents alkyl or alkenyl groups of 1 to 5 carbon atoms; alkoxy, alkylthio, or amido groups of 1 to 5 carbon atoms; aryl, alkaryl, or aralkyl groups of up to 6 carbon atoms; aryloxy, arylthio, or anilino groups of up to 6 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups of up to 6 ring carbon atoms; or fused ring or bridging groups of up to 14 ring atoms.
 14. A thermographic element comprising a support bearing at least one, image-forming, thermographic emulsion layer comprising:(a) a non-photosensitive, reducible silver source; (b) a reducing agent system for silver ion; and (c) a binder;wherein said reducing agent system comprises: (i) at least one hindered phenol; (ii) at least one co-developer of the formula

    R.sup.1 --(C═O)--NHNH--R.sup.2

wherein:R¹ represents hydrogen and R² represents an aryl group; or, R¹ represents hydrogen, alkyl or alkenyl groups of 1 to 20 carbon atoms; alkoxy, thioalkoxy, or amido groups of 1 to 20 carbon atoms; aryl, alkaryl, or aralkyl groups of up to 20 carbon atoms; aryloxy, thioaryloxy, or anilino groups of up to 20 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups comprising up to 6 ring carbon atoms; or fused ring or bridging groups comprising up to 14 ring atoms; and R² represents a trityl group; and (iii) at least one N-acyl-hydrazine compound of the formula ##STR12## wherein R³ represents an alkyl or alkenyl groups of 1 to 20 carbon atoms; alkoxy, thioalkoxy, or amido groups of 1 to 20 carbons; aryl, alkaryl or aralkyl groups comprising up to 20 carbon atoms; aryloxy, thioaryloxy, or anilino groups of up to 20 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carbocyclic ring groups comprising up to 6 ring carbon atoms; or fused ring or bridging groups comprising up to 14 ring atoms.
 15. The thermographic element according to claim 14 wherein said co-developer is a mixture of at least one trityl hydrazide and at least one formylphenyl hydrazine.
 16. The thermographic element according to claim 16 wherein said binder is hydrophobic.
 17. The thermographic element according to claim 14 wherein R¹ represents hydrogen and R² represents phenyl.
 18. The thermographic element according to claim 14 wherein R¹ represents hydrogen or an α-thioether group and R² represents CPh₃.
 19. The thermographic element of claim 14 wherein said hindered phenol is selected from the group consisting of binaphthols, biphenols, bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)methanes, and hindered naphthols. 